1. Field of the Invention
The present invention relates to a semiconductor memory device including resistance change elements having a chain structure, and write and read methods of the same.
2. Description of the Related Art
Recently, a number of memories that store information by new principles have been proposed. An MRAM (Magnetic Random Access Memory) using the TMR (Tunneling Magneto Resistive) effect is one of these memories.
A memory cell of this MRAM comprises, e.g., an MTJ (Magnetic Tunnel Junction) element and switching element. Letting F (Feature size: a minimum processing dimension) be the short side of the MTJ element and 2 F be its long side, the cell size is 12 F2 when a MOSFET is used as the switching element. This cell size is larger than that of a DRAM or flash memory. Therefore, a method that forms a bit line for easy axis write below the MTJ element and forms the lower electrode and the fringe of this contact in self-alignment is sometimes used. In this case, the cell size decreases to 10 F2, but this cell size is still unsatisfactory for micropatterning.
To achieve a 256-Mbit class, large-capacity MRAM, it is necessary to decrease the cell area to about 1 μm2 or less and downsize a peripheral circuit of the cell. To decrease the cell area to about 1 μm2 or less, F of the cell design rule must be decreased to about 0.13 μm. To miniaturize the peripheral circuit of the cell and decrease the ratio occupied by the cell to about 0.6, it is essential to decrease the value of a cell write current to about 1 mA or less. When the cell width F is about 0.4 μm, however, even the smallest write current value of the presently reported MTJ elements is about 8 to 10 mA (e.g., non-patent references 1 and 2).
In the recently reported spin injection type MRAM, an electric current is supplied perpendicularly to the film surface of the MTJ element, and spins are injected into a recording layer in accordance with the flowing direction of the electric current, thereby causing magnetization reversal. When the MTJ element for spin injection is a perpendicular magnetization type element, the element need only be given uniaxial anisotropy in the direction perpendicular to the film surface; the element need not be given shape magnetic anisotropy in the direction of the film surface unlike in a planar magnetization type (parallel magnetization type) element. Accordingly, it is in principle possible to miniaturize the MTJ element to its processing limit by setting its aspect ratio at 1. In addition, this MTJ element obviates the need for current magnetic field interconnections for generating current magnetic fields in different directions for two axes, unlike in a planar magnetization type element; the MTJ element can operate if only two terminals connecting to the upper and lower electrodes of the MTJ element exist. Consequently, the cell area per bit can be reduced (e.g., non-patent reference 3).
Unfortunately, to use one MOSFET as a switching element for one MTJ element, this MOSFET must have a size that makes it possible to supply an electric current necessary to cause magnetization reversal in the MTJ element. Also, the cell size is difficult to decrease because reducing the write current is an essential subject. Furthermore, the conventional cell structure makes it difficult to three-dimensionally stack MTJ elements. This makes the cell density difficult to greatly increase.
[Non-patent Reference 1] Roy Scheuerlein et al., “A 10 ns R and W Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest pp. 128-129
[Non-patent Reference 2] M. Durlam et al., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, 2000 IEEE
[Non-patent Reference 3] W. C. Jeong et al., “High scalable MRAM using field assisted current induced switching”, 2005 VLSI Sympo. Technical Digest pp. 184-185
[Non-patent Reference 4] S. Mangin et al., Nature Materials, Vol. 5, March 2006
[Patent Reference 1] Jpn. Pat. Appln. KOKAI Publication No. 2004-517504